#include<stdio.h>
#define dim 10001
#define li 28
using namespace std;

int lit[li], l[li];
char s[dim];

int main()
{ int n, i, j,cont,k,x;

	FILE *f = fopen("litere.in", "r");
	FILE *g = fopen("litere.out", "w");
	
	fscanf(f, "%d\n", &n);
	
	//fscanf(f, "%s", &s);
	//fgets(s, dim, f);
	for(i = 1; i <= n; i++)
		fscanf(f, "%c", &s[i]);
		//lit[(int)s[i] - 'a'+1]++;
	
	k = 0;
	for(i = 1; i <= n; i++)
		{
			x = (int)s[i] - 'a'+1;
			l[x]++;
			cont = 0;
			for(j = x+1; j <= li; j++)
				if(l[j] != 0) cont+=l[j];
			//aux = s[i], s[i] = s[i-cont], s[i-cont] = aux;
			k += cont;
		}
	
	fprintf(g, "%d\n", k);
	
	fclose(f);
	fclose(g);
	return 0;
}